Horizontal wells are nowadays more often exercised in developing oil and gas fields because of their potential benefit of increasing the drainage area. Complex reservoir structures often impose the use of numerical simulation when interpreting the transient pressure behaviour of horizontal wells.
In this work a comprehensive investigation of the available models listed in the literature used for numerical simulation of transient pressure behaviour of horizontal wells was performed. Then, a sensitivity analysis focusing on the effect of grid size selection and the representation of horizontal wellbore within the grid blocks were carried out. We have highlighted the technique to simulate transient behaviours of horizontal wells with a conventional reservoir simulation code. In addition we made a comparison of simulated well tests to the analytical solutions. We observed that there is a limit to grid refinement beyond which the results do not improve. However, using the log-distributed grids with conventional well index, results in closer match with the analytical solution at early times.
Horizontal wells have been successful in increasing productivity in a cost effective manner in overall field operations. They are being used in variety of conditions: reducing water and gas conning, thin pay zones, heavy oil recovery, improving sweep, tapping un-swept oil and improving sweep. In enhanced oil recovery applications horizontal wells have been especially beneficial. Whether they are used as injectors or producers, their higher reservoir contact area improves the efficiency of the displacements and production. Forecasting the economic feasibility of horizontal wells requires a reliable and accurate representation of these wells in numerical models to predict their expected production performances.
Most of the transient pressure well tests in horizontal wells have been directed to determine the permeability, skin factor, and the effective producing length. To use pressure transient test data in computerized methods for integrated reservoir characterization, numerical simulations of the well tests are typically required. When numerically solving the equations for flow in a reservoir, the block pressure cannot be assumed to be equal to the well bore pressure. These two pressures can be related through a well model that requires a well index (WI). The WI depends on reservoir properties and well geometry. Peaceman1,2 has derived expressions for the WI under the conditions of two-dimensional flow for an isolated well under steady state or pseudo-steady-state conditions. The default procedure in most commercial simulators is to use Peaceman's model. For a pressure transient test the assumptions of steadystate incompressible flow are not applicable. Care is required to avoid numerical artifacts occurring in the simulation so that they do not adversely effect the reservoir characterization. To account for this it is common to use highly refined grids around wells and to make very careful choices of time-step sizes to model well tests.
In this work a comprehensive investigation of the available models listed in the literature used for numerical simulation of transient pressure behaviour of horizontal wells has been performed. Well tests simulated using finite difference simulation are compared to analytical well test solutions.
